1. Field of the Invention
The present invention relates to an image processing apparatus and a processing method thereof.
2. Description of the Related Art
In recent years, there has been strong demand for high-resolution color images, and in response to calls for improved quality of these images, digital multifunction peripherals now handle images with resolutions of 1200 dpi (dots per inch) or more. With image processing apparatuses such as digital multifunction peripherals, memory and hard disk space are saved and the write time to these memories is shortened, by performing encoding compression when internally spooling image data to be handled by the apparatus, allowing reductions in cost and increases in speed to be achieved.
Here, JPEG schemes utilizing the discrete cosine transform and schemes utilizing wavelet transform are often used as compression schemes for still image data handled by an image processing apparatus. These encoding schemes encode an image in prescribed block units (for example, 8×8 or 16×16 pixels), and obtain a high compression efficiency by performing the discrete cosine transform, quantization and entropy coding.
Also, Japanese Patent Laid-Open No. 2004-104621 discloses an image coding apparatus that divides an image into small block unit areas (for example, 2×2 pixels), determines the representative color and interpolation representative color of the pixels constituting the blocks, and performs encoding at a fixed compression ratio by converting the resolution.
However, with an image processing apparatus that performs image processing after decoding encoding-compressed data that has undergone resolution conversion, there is a problem in that the compression efficiency decreases as shown below.
With an image processing apparatus, a pseudo-gradation process such as color conversion, filtering or dithering and a smoothing process are performed when printing, after decoding image data that has been encoding-compressed and spooled, after which image forming is performed. With smoothing, for example, edge enhancement is performed using a pixel interpolation process on edge portions of text and lines, which may lead to a decrease in resolution due to pixels in the block unit areas on which encoding compression is performed becoming thicker as a result of this pixel interpolation process. That is, pixels constituting a block that is determined to have a high resolution and is encoded at a low compression ratio when resolution conversion encoding is performed may become thicker low-resolution pixels due to the smoothing process, resulting in the high resolution determination when performing resolution conversion encoding being pointless.